Always the Aliens come to kick our butts and take over, why? Can't Spielberg afford a better story? Well, Tom Hanks caracter in Saving Private Ryan was a school teacher?
The series has the same writer as Saving Private Ryan. Maybe Barry Pepper will show up with a sniper rifle. He already had experience fighting aliens - and killing his career - in Battlefield Earth.
Anyway, generally plausible motives for invasion include
"resupply underway" - ie something like how Navy ships used to stop on Pacific islands for water and fruit
Strategic value of the place - ala the island hopping campaign in WW2
A Chevauchee (Google it) or viking style raid because that's just what they (the invaders) do culturally.
Hunting - ala Predator. Look at all the backward places that great White hunters used to travel.
"Them is us"/they were here before - raptors evolved a civilization, left earth, came back, etc. Or the space Nazis that conquered the future have come back. (Which really happened, but it split the timeline....Eh, it's a long story.)
That we're wrong about how hard/easy is it to travel interstellar and that a technological base slightly better than ours, equal to, or maybe even inferior in some ways could do it.
Lots of amusing firearms, including at least one PKM.
I'm more interested in the aliens and why they came in the first place. Arisakas and Zeros are interesting, but why the Japanese got themselves into a bind and ended up bombing Pearl Harbor is a lot more interesting.
The Skitters/cooties - the lizard like aliens are probably another slave race. The overlords that program the mechs are bipedal and likely humanoid if not fully "human". Rather than the Redcoats, they're more like the Hessians. Or maybe like sepoys I suppose. Rather than being the "real" enemy, they'll turn out to be a potential ally and Noah Wylie will be the George Washington that forms a new alliance, etc.
The alien overlords want biologicals to use as slave labor and/or need them for something else. Food maybe... Shrug.